Electronically controllable brake booster

ABSTRACT

An electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from each other by a movable wall, a control valve arrangement which can be actuated by an electromagnetic actuation device, through which a pressure difference between the pressure chamber and the vacuum chamber can be adjusted, with the control valve arrangement, as a function of a current flowing through the electromagnetic actuation device, assuming a holding position in which the current ranges between a higher value and a lower value without the control valve arrangement leaving the holding position, a first pressure changing position in which the current is higher than the higher value, and a second pressure changing position in which the current is lower than the low value, characterised in that upon a changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or upon a changeover from the first or the second pressure changing position into the holding position, the current value is sensed at which the respective changeover actually takes place, and this sensed current value is utilised for the specification of the higher value or the lower value, respectively, of the currents defining the holding position for a future corresponding changeover of the control valve arrangement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No. PCT/EP98/06223 filed Sep. 30, 1998, which claims priority to German Patent Application No. 19744054.1 filed Oct. 6, 1997.

BACKGROUND OF THE INVENTION

The present invention relates to an electronically controllable brake booster. In particular, the present invention relates to an electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from each other by a movable wall, a control valve arrangement which can be actuated by means of an electromagnetic actuation means, and by means of which a pressure difference between the pressure chamber and the vacuum chamber can be adjusted, with the control valve arrangement, as a function of a current flowing through the electromagnetic actuation means, assuming a holding position in which the current ranges between a higher value and a lower value without the control valve arrangement leaving its holding position, a first pressure changing position in which the current is higher than the higher value, and a second pressure changing position in which the current is lower than the low value.

From DE 195 27 493 A1 an electromagnetic actuation means is known which comprises a solenoid coil which can be subjected to a control current and an armature which is associated with the solenoid coil and which is adapted to perform movements which are a function of a control current flowing through the solenoid coil and a spring arrangement which biases the armature in the opposite direction. Therein, a holding position is defined as a manipulated variable, which the armature assumes at a holding current through the solenoid coil. This holding current flowing through the solenoid coil can be changed to a higher value or to a lower value without the armature leaving the holding position.

In addition, both the higher and the lower current value are dimensioned in such a manner that interfering influences on the magnetic and spring forces actuating the armature do not bring the armature into an actuated position which differs from the holding position.

For this purpose, the higher current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined; the control current is increased by a predetermined current step in a stepwise manner and output as a manipulated variable to the solenoid coil of the electromagnetic actuation means, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the second actuated position.

The lower current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined; the control current is decreased by a predetermined current step in a stepwise manner, and output as a manipulated variable to the solenoid coil of the electromagnetic actuation means, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the first actuated position.

From this document is it also known that the control valve arrangement can reliably be brought into the holding position if the arithmetic mean value of the currents is selected for the holding current.

However, the “Learning of the working points” covered in DE 195 27 493 A1 is limited in that the decisive currents for the lower limit and the upper limit must be learned and stored at each commencement of a trip upon switching on the ignition or in periodic time intervals during driving. Upon controlling the control valve arrangement, the currents which have been learned in this manner are used for specifying the first manipulated variable in order to achieve an adequate control behaviour.

In addition, the pressure difference acting upon the movable wall of the brake booster, which can be adjusted to different values depending upon the desired control, is not considered quantitatively.

The pressure difference adjusted at the movable wall is also acting immediately upon the valve body, the valve seat, and the valve element, which also leads to a shift of the currents for the lower limit and for the upper limit.

SUMMARY OF THE INVENTION

The invention deals with the problem which results from the shift of the upper and lower current values that are necessary in order to retain the control valve arrangement in its holding position.

It is the object of the present invention to eliminate this disadvantage so that the control behaviour is further improved.

As a solution, the invention proposes that, upon a changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or upon a changeover from the first or the second pressure changing position into the holding position, the current value is sensed at which the respective changeover actually takes place, and this sensed current value is utilised for the specification of the higher and/or lower value of the currents defining the holding position for a future corresponding changeover of the control valve arrangement.

In a preferred embodiment of the invention, the first pressure changing position is a pressure build-up position and the second pressure changing position is a pressure relief position. Depending on the configuration of the control valve arrangement or its connection, respectively, with the vacuum chamber or pressure chamber, respectively, the reverse configuration is possible.

In an embodiment of the invention, the respective first current value (I) for the specification of the higher value (I_(AUFBAU)) or the lower value (I_(ABBAU)), respectively, of the currents defining the holding position is utilised for a future changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or in a future changeover from the first or the second pressure changing position into the holding position during a running control operation.

Alternatively, the respective sensed current value for the specification of the higher or lower, respectively, value of the currents defining the holding position can be utilised for a future changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or in a future changeover from the first or the second pressure changing position into the holding position in a future control operation.

In a further configuration, the respective sensed current value for the specification of the higher value or the lower value, respectively, of the currents defining the holding position is utilised within a weighted mean value generation with further sensed current values for the generation of an averaged current value.

Moreover, it can be advantageous that the respective sensed current value for the specification of the higher value or the lower value, respectively, of the currents defining the holding position is utilised within a sliding mean value generation with further sensed current values for the generation of an averaged current value.

In order to exclude excessive deviations from the standard case and still detect and account for gradual changes (caused by friction, wear, temperature influences, etc.), it is advantageous to check if the respective value lies within a predetermined tolerance band or is in agreement with a current change tendency, before the respective sensed current value for the specification of the higher value or the lower value, respectively, of the currents defining the holding position is utilised for the generation of an averaged current value.

Further properties, advantages, characteristics, and variation possibilities of the invention will be explained by means of the following description of a currently preferred embodiment of the invention with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a vehicle brake system with an electronically controllable brake booster.

FIG. 2 schematically shows a simple control loop for the operation of an electronically controllable brake booster.

FIG. 3 shows a schematic diagram for explaining the control characteristic of an electronically controllable brake booster.

FIG. 4 schematically shows a flow diagram for explaining the operation of the inventive electronically controllable brake booster.

DETAILED DESCRIPTION OF THE INVENTION

In the vehicle brake system which is schematically shown in FIG. 1, a brake pedal 1 is used to actuate a brake pressure generator unit 2 via an actuating element. The brake pressure generator unit 2 comprises a brake cylinder 25 in which a piston 28 defines a pressure chamber 29. The pressure chamber 29 is supplied with brake fluid from a reservoir 27. A brake line 3 leads from the pressure chamber 29 to a wheel brake 4 of the vehicle.

An anti-blocking control means and/or a traction control means ABS/TC are arranged in the brake line 3 between the brake pressure generator unit 2 and the wheel brake 4. The anti-blocking and/or traction control means ABS/TC comprises i.a. valve and pump arrangements in a known manner which are driven by an electronic control unit ECU in order to modulate the pressure in the wheel brake 4. This occurs as a function of the rotational behaviour of a vehicle wheel associated with the wheel brake 4, which is sensed by means of a sensor 41 and supplied to the electronic control unit ECU.

The brake pressure generator unit 2 comprises a brake booster 21 for the amplification of the actuation force which is introduced by the driver via the brake pedal 1. A movable wall 22 divides the brake booster 21 into a vacuum chamber 23 and a pressure chamber 24. For the generation of the vacuum, the vacuum chamber is connected to a vacuum source Vac which is not shown in detail. In a vehicle which is equipped with an Otto engine, the vacuum which is inherently produced in the intake tube, is available. However, in a vehicle which is powered by a Diesel engine or an electric motor, an additional vacuum pump is required as the vacuum source Vac. Upon an actuation of the brake pedal 1, the brake booster functions in a known manner in that the pressure chamber 24 is subjected to atmospheric pressure so that a pressure difference is acting on the movable wall 22, which assists the actuation force introduced at the brake pedal 1. In the non-actuated condition the vacuum chamber 23 and the pressure chamber 24 are connected with each other and thus pressure compensated so that no pressure difference is effective at the movable wall 22.

The brake booster 21 is also electronically controllable via a solenoid arrangement 26. The electronic controllability of the brake booster 21 makes it possible to carry out braking operations also automatically, i.e. independent of an actuation of the brake pedal 1. This can be utilised, for example, for the realisation of a traction control, driving dynamics control, or vehicle-to-vehicle ranging control. A sensor means 11 is provided in order to sense parameters which are related to the actuation of the brake pedal 1, such as, for example, pedal travel, pedal force, or pedal actuation speed for the evaluation in the electronic control unit ECU, in order to also carry out braking operations in emergency situations, with, for example, exceeding a certain pedal actuation speed serving as the criterion.

For this purpose, the solenoid arrangement 26 actuates a control valve which is not shown in detail herein, in order to bring the brake booster 21 into different control positions (I,. II., III.):

into a first so-called “build-up” position (I.) in which the connection of the vacuum chamber 23 with the pressure chamber 24 is blocked, and the connection of the pressure chamber 24 with atmosphere is open so that a pressure difference at the movable wall 22 is built up or increased, respectively,

or into a second so-called “holding position” (II.) in which the connection of the vacuum chamber 23 with the pressure chamber 24 and the connection of the pressure chamber 24 with atmosphere are blocked so that a pressure difference acting on the movable wall is maintained,

or into a so-called “relief position” (III.) in which the connection of the vacuum chamber 23 with the pressure chamber 24 is open, and the connection of the pressure chamber 24 with atmosphere is blocked so that a pressure difference acting on the movable wall 22 is relieved via a pressure compensation process.

In order to bring the control valve for a modulation of the pressure difference at the movable wall 22 into the different control positions (I., II., III.) the electronic control unit ECU supplies a control current I_(SOLL) to the solenoid arrangement 26, with the variation of the control current I_(SOLL) being effected, for example, by means of pulse width modulation. A magnetic force is acting on the armature of the solenoid arrangement 26, which causes a positioning of the armature, according to which the control positions (I., II., III.) result.

The brake pressure p_(IST) which is generated in the pressure chamber 29 of the brake cylinder 25 and introduced into the brake line 3 is sensed by means of a sensor 31 and transmitted to the electronic control unit ECU in order to control the brake pressure p_(IST) as a function of the desired pressure value and/or the pressure characteristic p_(SOLL) by adjusting the control current I_(SOLL) which is supplied to the solenoid arrangement 26.

The operation of the electronically controllable brake booster 21 in a closed control loop is shown in FIG. 2. A controlled variable, the brake pressure p_(IST) generated in the brake cylinder 25, which originates from the controlled system, the brake pressure generator unit 2, is continuously sensed and compared with a reference variable, the desired pressure characteristic p_(SOLL). The result of this comparison is a standard deviation x_(d) which is supplied to a controlling means R, whereby the controlling means R can be part of the electronic control unit ECU. The manipulated variable which originates from the controlling means R is the control current I_(SOLL) which is supplied to the solenoid arrangement 26. The interfering variables z are mainly influences caused by friction losses, tolerances, temperature variations, or variations of external reaction forces which, in particular, include variations of the pressure force component in the vacuum chamber 23 of the brake booster 21.

In the diagram according to FIG. 3, the travel is plotted over the abscissa and the magnetic force which acts on the armature and which results as a function of the control current I_(SOLL) is plotted over the ordinate. This is an idealised schematic representation which relates to a working range which is designed in such a manner that a proportional relationship exists between magnetic force and control current. In addition, the control characteristic of the electronically controllable brake booster 21 is entered. This control characteristic comprises a total of three branches. For the vertical branch, a current range I_(ABBAU)<I_(SOLL)<I_(AUFBAU) is associated with a certain armature position x₀, with the position x₀ exactly corresponding to the holding position (II.). The inclined branch extending to the left from the vertical branch applies to a current I_(SOLL)>I_(AUFBAU) and represents the build-up position (I.), while the inclined branch extending to the right of the vertical branch applies to a current I_(SOLL)<I_(ABBAU) and characterises the relief position (III).

Due to the already mentioned interfering influences which are caused by friction, tolerances, reaction forces, etc. the control characteristic in the area of the inclined branches has a scatter band, which leads to a shift of the working points I_(ABBAU) and I_(AUFBAU). In order to counteract this problem with respect to the holding position (II.), a current is preferably adjusted for I_(HALT), which results as the arithmetic mean value of the currents I_(ABBAU) and I_(AUFBAU). This and in particular a method for learning the currents I_(ABBAU) and I_(AUFBAU) which determine the working points is known from DE 195 27 493 A1.

Disadvantageously, the method covered in DE 195 27 493 A1 is restricted to learning and storing the currents I_(ABBAU) and I_(AUFBAU) merely at each commencement of a trip upon switching on the ignition or in periodic time intervals during driving, provided there is no control request for the electronically controllable brake booster 21. When there is a control request for the electronically controllable brake booster 21, the current values which have been learned in this way are used in each changeover from the holding position (II.) into the relief position (III.) as well as in each changeover from the holding position (II.) into the build-up position (I.) for specifying the first manipulated variable, in order to achieve an adequate control behaviour. If, by the above mentioned interfering influences, a shift of the working points I_(ABBAU) and I_(AUFBAU) is caused during the time period for which a control request for the electronically controllable brake booster 21 is applied, a sluggish control behaviour can result at each changeover from the holding position (II.) into the relief position (III.) or into the build-up position (I.), respectively, so that the performance of the electrically controllable brake booster 21 as a whole deteriorates. This is true in particular in that case in which the control request for the electronically controllable brake booster 21 is applied for a relatively long time, which can certainly be the case when performing a traction control, a driving dynamics control, or a vehicle-to-vehicle ranging control.

In order to improve the performance of the electronically controllable brake booster, it is therefore desired to eliminate this drawback.

The inventive method is, initially, based on sensing the actually decisive current value I_(AUFBAU,IST) for the changeover from the holding position (II.) into the build-up position (I.) and/or the actually decisive current value I_(ABBAU,IST) for the changeover from the holding position (II.) into the relief position (III.) during a control in each cycle in which, starting from the holding position (II.), the build-up position (I.) and/or, starting from the holding position (II.), the relief position (III.) is assumed, in order to determine as a function of same the higher and/or lower current value I_(AUFBAU,NEU), I_(ABBAU,NEU) defining the holding position (II.) for the next cycle or the next control.

In order to avoid ambiguities, it should be noted here that the term “control” used herein relates to the time period during which a control request for the electronically controllable brake booster 21 is present, and the term “cycle” relates to the adjustment of a build-up, holding, or relief position (I., II., III.) for the purpose of modulating the pressure difference at the movable wall 22 within a “control”.

Advantages and configuration possibilities will be explained in the following with reference to an embodiment. For this purpose, a flow diagram is presented in FIG. 4.

The flow diagram according to FIG. 4 is cyclically executed. Therein, N_(AUFBAU) designates a counter which counts the number of changeovers from the holding position (II.) into the build-up position (I.) during a control, and N_(ABBAU) designates a counter, which counts the number of changeovers from the holding position (II.) into the relief position (III.). S_(AUFBAU) designates a variable in which the decisive current values I_(AUFBAU) for the changeover from the holding position (II.) into the build-up position (I.) during a control are added up. Correspondingly, in S_(ABBAU) the decisive current values I_(ABBAU) for the changeover from the holding position (II.) into the relief position (III.) during a control are added up. If a control is not active or terminated, respectively, an initialisation or re-initialisation takes place in such a manner that the counters N_(AUFBAU) and N_(ABBAU) are reset and the summated variables S_(AUFBAU) and S_(ABBAU) are set to zero again.

If a control is active, a distinction is first made whether the current cycle is a changeover from the holding position (II.) into to build-up position (I.) or a changeover from the holding position (II.) into the relief position (III.). If neither of the two cases applies, the run is aborted and continued with the next cycle.

For the distinction, whether a changeover from the holding position (II.) into the build-up position (I.) or a changeover from the holding position (II.) into the relief position (III.) is involved, the standard deviation x_(d) is evaluated. If the standard deviation is negative (x_(d)<0), i.e. the brake pressure p_(IST) is lower than the desired reference variable p_(SOLL), a changeover from the holding position (II.) into the build-up position (I.) takes place. If, however, the standard deviation is positive (x_(d)>0), i.e. the brake pressure p_(IST) is higher than the desired reference variable p_(SOLL), a changeover from the holding position (II.) into the relief position (III.) takes place.

Upon a changeover from the holding position (II.) into the build-up position (I.) the counter N_(AUFBAU) is incremented and the actually decisive current value I_(AUFBAU,IST) for the changeover from the holding position (II.) into the build-up position (I.), which was previously sensed, is added to the summated variable S_(AUFBAU). Subsequently, the summated variable S_(AUFBAU) is divided by the counter N_(AUFBAU) and the result is stored as new higher current value I_(AUFBAU,NEU) defining the holding position (II.).

In the changeover from the holding position (II.) into the relief position (III.) the procedure is analogous. Here, the counter N_(ABBAU) is incremented and the actually decisive current value I_(ABBAU,IST) for the changeover from the holding position (II.) into the relief position (III.) is added in the summated variable S_(ABBAU). The result of the division of the summated variable S_(ABBAU) by the counter N_(ABBAU) is then stored as new lower current value I_(ABBAU,NEU) defining the holding position (II.).

The further processing of the higher or lower, respectively, new current values I_(AUFBAU,NEU), I_(ABBAU,NEU) defining the holding position (II.) which have been determined in this manner can be done by various methods.

Firstly, the new higher current value I_(AUFBAU,NEU) defining the holding position (II.) which is determined in the current cycle, can be specified already in the following cycle, wherein a changeover from the holding position (II.) into the build-up position (I.) takes place, or the new lower current value I_(ABBAU,NEU), respectively, defining the holding position (II.), which is determined in the current cycle, can be specified already in the following cycle, wherein a changeover from the holding position (II.) into the relief position (III.) takes place, as the first manipulated variable. In this way, a continuous and thus very dynamic adjustment to shifts of the working points I_(AUFBAU) or I_(ABBAU), respectively, due to interfering influences, is achieved.

Secondly, the actually decisive current values I_(AUFBAU,IST) for the changeover from the holding position (II.) into the build-up position (I.) or the actually decisive current values I_(ABBAU,IST), respectively, for the changeover from the holding position (II.) into the relief position (III.) can first be observed for a predetermined learning phase after entering a control, in order to specify the new higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) as the first manipulated variable only following this learning phase. Then, there are the options to either retain the new higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) defining the holding position (II.), which has been determined in the learning phase at the start of the control, unchanged until the end of the control as the first manipulated variable, or to have several learning phases executed successively during the control and to specify the new higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) determined thereby as the respective first manipulated variable. It is true that compared to the first mentioned method, a somewhat less dynamic adjustment to shifts of the working points I_(AUFBAU) or I_(ABBAU), respectively, due to interfering influences, takes place, however, possible inaccuracies in the determination of the actually decisive current values I_(AUFBAU,IST) for the changeover from the holding position (II.) into the build-up position (I.), or the actually decisive current values I_(ABBAU,IST) for the changeover from the holding position (II.) into the relief position (III.) are compensated.

The flow diagram according to FIG. 4 shows the possibility to determine the duration of the learning phase via the number of changeovers from the holding position (II.) into the build-up position (I.), or the number of changeovers from the holding position (II.) into the relief position (III.), respectively. Therein, the counter N_(AUFBAU) or N_(ABBAU), respectively, is compared with a predeterminable limit N_(LIMIT). If the counter N_(AUFBAU) or N_(ABBAU), respectively, exceeds the limit, the learning phase is considered terminated, whereupon the run is aborted and continued with the next cycle.

Thirdly, provisions can be made that then when a control is not active or is terminated, respectively, the new higher or lower, respectively, current value(s) I_(AUFBAU,NEU), I_(ABBAU,NEU) which was (were) (last) determined during the previous control(s) is weighted with the new higher or lower, respectively, current value(s) I_(AUFBAU,NEU), I_(ABBAU,NEU) which was (were) (last) determined during the previous control(s) in order to determine a higher or lower, respectively, current value I_(AUFBAU,INIT), I_(ABBAU,INIT) as the initialisation value for the next control. The initialisation value determined in this manner is then set in the first cycle of the following control.

The weighting can be effected, for example, in the form of a difference equation over a certain number of higher or lower, respectively, current values I_(AUFBAU,NEU), I_(ABBAU,NEU) of the previous controls. If, for example, the weighting is effected over 10 previous controls, with simply performing a mean value generation, then the higher or lower, respectively, current values I_(AUFBAU,NEU), I_(ABBAU,NEU) from the previous controls are weighted each by the factor {fraction (1/10)}.

I _(AUFBAU,INIT)={fraction (1/10)}*[I _(AUFBAU,NEU) ^((n)) +I _(AUFBAU,NEU) ^((n−1)) +. . . +I _(AUFBAU,NEU) ^((n−9))], or

I _(ABBAU,INIT)={fraction (1/10)}*[I _(ABBAU,NEU) ^((n)) +I _(ABBAU,NEU) ^((n−1)) +. . . +I _(ABBAU,NEU) ^((n−9))], respectively.

However, it can also be provided to effect the weighting with different factors. If weighting is done, for example, over 2 previous controls in such a manner that the higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) from the last but one control is weighted by the factor {fraction (9/10)}, and the higher or lower, respectively, current values I_(AUFBAU,NEU), I_(ABBAU,NEU) from the last control is weighted by the factor {fraction (1/10)}.

I _(AUFBAU,INIT)={fraction (1/10)}*I _(AUFBAU,NEU) ^((n))+{fraction (9/10)}*I _(AUFBAU,NEU) ^((n−1)), or

I _(ABBAU,INIT)={fraction (1/10)}*I _(ABBAU,NEU) ^((n))+{fraction (9/10)}*I _(ABBAU,NEU) ^((n−1)), respectively.

Due to the fact that the higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) from the last but one control is higher weighted than the higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) from the last control, the advantage is given that a unique “maverick” does not negatively influence the determination of the higher or lower, respectively, initialisation value I_(AUFBAU,INIT), I_(ABBAU,INIT), i.e. that a severe abrupt change of the higher or lower, respectively, initialisation value I_(AUFBAU,INIT), I_(ABBAU,INIT) is avoided.

Under practical conditions it has proven particularly advantageous to make the weighting during the first 10 controls with identical factors, i.e. {fraction (1/10)}, and then to go to a weighting covering 2 controls with two different weighting factors, i.e. for example, {fraction (9/10)} and {fraction (1/10)}.

It should be pointed out here that the weighting proposed in this context can, of course, also be applied in a similar manner in each determination of the new higher or lower, respectively, current value I_(AUFBAU,NEU), I_(ABBAU,NEU) defining the holding position (II.)

Finally, the following advantages which are obtained in the inventive operation of the electronically controllable brake booster will be emphasised.

On the one hand, the optimum determination of the working points I_(AUFBAU) or I_(ABBAU), respectively, results in shorter response times at each changeover from the holding position (II.) into the build-up position (I.) or into the relief position (III.), respectively, which improves the performance of the electronically controllable brake booster in its entirety rather significantly.

On the other hand, the learning capability enables the automatic detection and correction of shifts of the working points I_(AUFBAU) or I_(ABBAU), respectively, so that negative influences of long-term effects are eliminated. This is of importance, in particular, in the case of a replacement of the electronic control unit ECU or the brake booster 21, because subsequently thereto the correct working points I_(AUFBAU) or I_(ABBAU), respectively, are adjusted automatically.

In addition, it is understood that the inventive operation of the electronically controllable brake booster with respect to noise generation and accuracy is performed particularly comfortable.

In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. 

What is claimed is:
 1. An electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from each other by a movable wall, a control valve arrangement which can be actuated by means of an electromagnetic actuation means, and by means of which a pressure difference between the pressure chamber and the vacuum chamber can be adjusted, with the control valve arrangement, as a function of a current value flowing through the electromagnetic actuation means, assuming a holding position in which the current ranges between a higher value and a lower value without the control valve arrangement leaving the holding position, a first pressure changing position in which the current is higher than the higher value, and a second pressure changing position in which the current is lower than the low value, characterised in that by means of suitable means during a control upon each changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or upon each changeover from the first or the second pressure changing position into the holding position, the current value is sensed at which the respective changeover actually takes place, and this sensed current value is utilised for the specification of one of the higher value and the lower value, respectively, of the currents defining the holding position for a future corresponding changeover of the control valve arrangement.
 2. The electronically controllable brake booster according to claim 1, characterised in that the first pressure changing position is a pressure build-up position, and the second pressure changing position is a pressure relief position.
 3. The electronically controllable brake booster according to claim 1, characterised in that by means of the suitable means the respective sensed current value for the specification of one of the higher value and the lower value, respectively, of the currents defining the holding position is utilised for a future changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or from the first or the second pressure changing position into the holding position during a running control operation.
 4. The electronically controllable brake booster according to claim 1, characterised in that by means of the suitable means the respective sensed current value for the specification of one of the higher value and lower value, respectively, of the currents defining the holding position can be utilised for a future changeover of the control valve arrangement from the holding position into the first or the second pressure changing position, or for a changeover from the first or the second pressure changing position into the holding position after termination of the running control operation in a future control operation.
 5. The electronically controllable brake booster according to claim 1, characterised in that by means of the suitable means the respective sensed Current value for the specification of one of the higher value and lower value, respectively, of the currents defining the holding position is utilised within a weighted mean value generation with further sensed current values for the generation of an averaged current value.
 6. The electronically controllable brake booster according to claim 1, characterised in that by means of the suitable means the respective sensed current value for the specification of one of the higher value and lower value, respectively, of the currents defining the holding position is utilised within a sliding mean value generation with further sensed current values for the generation of an averaged current value.
 7. The electronically controllable brake booster according to claim 1, characterised in that by means of the suitable means it is checked whether the respective sensed current value lies within a predetermined tolerance band or is in agreement with a current change tendency, before the respective sensed current value for the specification of one of the higher value and lower value, respectively, of the currents defining the holding position is utilised for the generation of an averaged current value. 